1. Field of the Invention
The invention generally relates to the operation of a User Equipment (UE) after it finishes a Circuit-Switched Fallback (CSFB) call, and more particularly, to apparatuses and methods for assisting a UE to camp back to a Long Term Evolution (LTE)-based network after finishing a CSFB call with a non-LTE network.
2. Description of the Related Art
With growing demand for ubiquitous computing and networking, various wireless technologies have been developed, such as the Wireless Local Area Network (WLAN) technologies, including the Wireless Fidelity (WiFi) technology, Bluetooth technology, and the Zigbee technology, etc., and also, the cellular technologies, including the Global System for Mobile communications (GSM) technology, General Packet Radio Service (GPRS) technology, Enhanced Data rates for Global Evolution (EDGE) technology, Wideband Code Division Multiple Access (WCDMA) technology, Code Division Multiple Access 2000 (CDMA2000) technology, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) technology, Worldwide Interoperability for Microwave Access (WiMAX) technology, Long Term Evolution (LTE) technology, Time-Division LTE (TD-LTE) technology, and LTE-Advanced technology, etc.
For user convenience and flexibility, most User Equipments (UEs) (or may be referred to as Mobile Stations (MSs)) nowadays support more than one wireless technology. Taking a UE supporting both of the WCDMA technology and the LTE technology for example, it may selectively obtain wireless services using the WCDMA technology or the LTE technology. Generally, it selects an LTE network over a Universal Mobile Telecommunications System (UMTS) network when wireless services are available from both of the service networks, since the LTE network may more likely provide wireless services with higher bandwidth than the UMTS network. Nonetheless, in some cases, a technique called Circuit-Switched Fallback (CSFB) may be employed for a UE which is already camped on an LTE network to switch to the UMTS network for accessing, particularly, Circuit-Switched (CS) services, e.g., a CS call, instead. The CS call made utilizing the CSFB technique is generally referred to as a CSFB call.
According to the 3GPP TS 25.331 specification, v10.2.0, if there exists an ongoing Packet-Switched (PS) session between the UE and the UMTS network after finishing the CSFB call, the UE may camp back to the LTE network only when receiving a specific message from the UMTS network, such as a Radio Resource Control (RRC) Connection Release message, or a Handover from UTRAN message. However, the UMTS network may not deploy the specific message, possibly because the operator of the UMTS network may want to keep the UE in the UMTS network. As a result, the UE may stay in the UMTS network and not be able to camp back to the LTE network to obtain, preferred, faster wireless services.